The present invention relates to a transmissively readable photoplastic film on which can be recorded erasable rippled message images. Photoplastic film finds use in both fiche and roll configurations for microfiche and microfilm recording. While the present invention is not limited thereto, it is directed primarily to providing an improved microfiche element.
In U.S. Pat. No. 3,147,062 issued Sept. 1, 1964 for "Medium For Recording", there is described a recording medium in the form of a film having preferably a 4 mil thick base of an optical grade of polyethylene terephthalate on which a conductive film of chromium is vacuum evaporated to a thickness of approximately 35 Angstroms. Onto this conductive film, there is bonded a thermoplastic layer having a thickness of about 7 microns or 0.32 mils of medium molecular weight polystyrene prepared from equal parts of types PS 1 and PS 2 resin produced by the Dow Chemical Company. The type PS 1 polystyrene is stated to have "a molecular weight of about 2,000 (extrapolated from manufacturer's data) and PS 2 has a molecular weight of about 16,500 (measured by osmotic pressure in chloroform)." However, the patent also states that by "medium molecular weight" is meant a molecular weight in the range of 10,000 to 25,000. Although the foregoing is presumably preferred, the patent states that the thermoplastic layer may have a thickness ranging between 0.01 mils and several mils. Additionally, it is disclosed that copper iodine may be used instead of chromium to produce a coating of 150 to 300 Angstroms thick. The conductive layer should have a resistance equal to about 1,000 ohms per square.
In U.S. Pat. No. 3,809,473 issued May 7, 1974 for "Method Of Reproducing A Relief Image" there is described in Example I a method for producing a photoplastic structure by preparing a coating from a mixture of one gram of copper phthalocyanine, 5 grams of low molecular weight poly-2-methylstyrene, (e.g., 276 V 9, marketed by Dow Chemical Company) and 10 grams of polystyrene having an average molecular weight of approximately 30,000, (e.g., PS 3, marketed by Dow Chemical Company).
A photoplastic film formed from a low molecular weight thermoplastic material is described in U.S. Pat. No. 4,077,803 issued Mar. 7, 1978 for "Low Charge-Voltage Frost Recording On A Photosensitive Thermoplastic Medium." More specifically, there is described a recording medium in which the substrate is formed from a commercially available polyester material such as polyethylene terephthalate sold under the "MYLAR" trademark by E. I. du Pont de Nemours and Company. The polyester material has bonded to one surface a thin coating of an electrically conductive material. The patent sets forth as an example a thin coating of sputtered or evaporated aluminum. Onto the conductive coating is bonded by spraying, dipping, or otherwise coating, a layer of a commercial grade of polystyrene thermoplastic resin. The patent states that one such material is commercially available from the Pennsylvania Industrial Chemical Corporation of Clairton, Pa. under the trade name "Piccolastic A-75". Such material is stated to be characterized by an estimated molecular weight of the order of 400 and to have a 75.degree. C. ball-and-ring softening point. As an alternative, there is mentioned a product of Hercules, Inc., of Wilmington, Del. sold under the trade name "Staybelite Ester 10" and which is a glycerol ester of hydrogenated resin. In this particular composition, copper phthalocyanine is added in a quantity of about 8 percent. The thickness of the thermoplastic layer is stated to be adjusted from 3 to 8 microns although the patent states elsewhere that layers varying in thickness from 1.2 to 7 microns are found useful.
In copending U.S. patent application Ser. No. 873,932 filed Jan. 31, 1978 for "Photoplastic Film Recording and Monitoring Apparatus", there is described and claimed a combination recording and monitoring machine for use with the transparent form of photoplastic microfiche. A typical fiche has a rectangular configuration measuring about 105 mm. long by 148 mm. wide with 98 message image-bearable panel or frame areas 12.5 mm. long by 10 mm. wide in a rectilinear pattern of seven transverse or lateral rows with fourteen such panel or frame areas in each row. Because of the large magnification involved, the film must be capable of high resolution, and the machine must afford accurate control of the optics for establishing sharp focus of projected images. It should be readily understood that an important requirement for the fiche is that it should lay flat and not tend to curl. Curling interferes with handling of the fiche and can detract from image quality.
An important requirement of a recording process is permanence except when deliberate erasure is desired. It was discovered, however, that a photoplastic film produced from a formulation of the type described in U.S. Pat. No. 3,809,473 suffered a 25 percent decrease in image contrast retention when exposed to a temperature of 40.degree. C. after a mere 220 hours. Fortunately, a reduction in the quantity of plasticizer, i.e., the alpha-methylstyrene resin, to 25 percent extends the time for equivalent image reduction from 220 hours to 4,600 hours at the same temperature exposure. Extrapolating it can be shown that at 20.degree. C. and a plasticizer concentration of 25 percent the film should survive about 160,000 hours before experiencing a 25 percent reduction in image contrast.
Unfortunately, the high molecular weight polystyrene type PS 3 is no longer manufactured. A polymer of lower molecular weight but higher softening point has been substituted. In combination with 25 percent plasticizer there is produced a film of comparable image producing quality. However, the substitution of resin and/or reduction of plasticizer was accompanied by a manifestation of a tendency of the film to curl, with its attendant drawbacks.